The performance improvement of electronic devices such as bipolar transistors and field-effect transistors has dramatically increased the operating speed of large-scale integrated circuits (LSI). However, despite the speed-up of the internal operation of LSI, the operating speed at the level of the printed circuit board on which the LSI is mounted is set lower than the operating speed of the LSI, and the operating speed at the level of the rack on which the printed circuit board is installed is set even lower.
These are attributed to the increase of transmission loss, noise, and electromagnetic interference associated with the increase of operating frequency, because a longer interconnect requires a lower operating frequency in order to avoid degradation in signal quality. Hence, in electrically interconnected apparatuses, even if the operating speed of LSI, which is an active device, is increased, the speed is unfortunately forced to decrease when mounted. Thus, a recent growing trend is that mounting technology is more dominant in system speed than LSI operating speed.
In view of the foregoing problem with electrically interconnected apparatuses, some proposals have been made for an optically interconnected apparatus, which includes optical interconnection between LSIs. An optically interconnected apparatus is characterized by having little frequency dependence of loss in the frequency range from DC to 100 GHz or more, and being free from electromagnetic interference with the interconnect path and from noise due to ground potential difference. Thus, interconnection at several 10 Gbps can be easily realized. Hence, the optically interconnected apparatus can be expected to operate at very high speed even at the level of the printed circuit board and rack.
In particular, because of its flexibility, an optoelectric composite interconnect of the flexible type provides a high degree of freedom in board mounting. Furthermore, the configuration is simplified because it includes electrodes for mounting an optical device in itself (e.g., JP-A 2003-227951 (Kokai)).